Polycrystalline translucent alumina sintered body, a method for producing the same and a high pressure vapor discharge lamp obtained by using said sintered body

ABSTRACT

A polycrystalline translucent alumina sintered body having high in-line transmission, which is suitable for an envelope for a high pressure vapor discharge lamp, is produced by adding to alumina powder having a purity of more than 99.8% MgO or a compound forming MgO in an amount of 0.01-0.1% by weight calculated as MgO, La 2  O 3  or a compound forming La 2  O 3  in an amount of 0.001-0.05% by weight calculated as La 2  O 3 , and Y 2  O 3  or a compound forming Y 2  O 3  in an amount of 0.001-0.5% by weight calculated as Y 2  O 3 , thoroughly mixing the resulting mixture, shaping the mixture into a desired form and firing the shaped article under vacuum or either atmosphere of hydrogen gas or dissociated ammonia gas under oxygen concentration of 10 -15  -10 -25  atm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polycrystalline translucent aluminasintered body having an excellent in-line transmission, a method forproducing the same and a high pressure vapor discharge lamp obtained byusing said alumina sintered body. The polycrystalline translucentalumina has excellent light transmission, heat resistance, corrosionresistance and the like, so that such an alumina sintered body has beenbroadly used for an envelope of a high pressure vapor discharge lamp,window for high temperature, memory erasing window and the like.

2. Description of the Prior Art

As a method for producing such polycrystalline translucent alumina, ithas been known that about 0.1% of MgO is added to fine, high purityalumina powder as an additive to control exaggerated grain growth ofalumina crystal grains, whereby a dense polycrystalline translucentalumina sintered body having substantially no pores is produced. Howeverthis method has had the defect that firing at a high temperature ofhigher than 1,800° C. is necessary for obtaining a sintered body havingan excellent transmission. A process wherein about 0.05-0.1% of each ofY₂ O₃ and La₂ O₃ is added as an additive in addition to MgO has beendisclosed but even though the firing temperature is lowered to1,650°-1,750° C. owing to the function of Y₂ O₃, La₂ O₃ and the like,the effects of these additives influencing the grain growth of aluminaparticles are different. Thus, alumina crystal grains in the obtainedsintered body are apt to be grown nonuniformly depending upon the ratioof MgO, La₂ O₃ and Y₂ O.sub. 3 added and therefore it cannot be avoidedto degrade the mechanical strength and thermal shock resistance.Furthermore, since amounts of MgO, La₂ O₃, Y₂ O₃, etc., added arelarger, different phases are formed in the crystal grain boundary andthe in-line transmission is very poor and there have been many defects.

SUMMARY OF THE INVENTION

The present invention has been made for obviating these defects andcomprises a polycrystalline translucent alumina sintered body in whicheach content of MgO, La₂ O₃ and Y₂ O₃ is 0.001-0.05% by weight ("%"means "% by weight" hereinafter), a weight ratio of MgO/La₂ O₃ +Y₂ O₃ is0.5-2 and the in-line transmission is more than 40%; a method forproducing the same; and a high pressure vapor discharge lamp whereinsaid alumina sintered body is used as an envelope. In the presentinvention, since MgO, La₂ O₃ and Y₂ O₃ coexist as the additive, thefiring can be carried out at a temperature as low as 1,650°-1,850° C.and since each content of the additive is as low as 0.001-0.05% and theadditive has the specific ration, the second phase is not substantiallyformed in alumina grain boundary, so that translucent alumina havingparticularly excellent transmission, mechanical strength and thermalshock resistance can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polycrystalline alumina sintered body according to the presentinvention can be produced by the following steps. To γ-alumina powderhaving a purity of more than 99.8% and a grain size of 0.01-0.1 μm areadded MgO or a compound forming MgO in an amount of 0.01-0.1% calculatedas MgO, La₂ O₃ or a compound forming La₂ O₃ in an amount of 0.001-0.05%calculated as La₂ O₃, and Y₂ O₃ or a compound forming Y₂ O₃ in an amountof 0.001-0.05% calculated as Y₂ O₃ and the resulting mixture is mixed inwet process in a ball mill for more than 10 hours and then taken outfrom the ball mill and dried and calcined at a temperature of1,150°-1,250° C. for 3-7 hours in air. As alumina starting material, usemay be made of α-alumina having a purity of more than 99.8% and a grainsize of 0.1-0.5 μm other than γ-alumina and in this case, the calciningstep is not necessary. To the calcined powder is added homogeneously1-3% of polyvinyl alcohol as a binder and the mixture is molded underhydraulic pressure of 5-30 kg/mm² into a desired shape. The thus shapedarticle is maintained at about 700° C. for about one hour in air to burnand remove the binder and then fired under vacuum or either atmosphereof hydrogen gas or dissociated ammonia gas and under an oxygenconcentration of 10⁻¹⁵ -10⁻²⁵ atm. The firing is preferred to be carriedout in two steps and the primary firing is carried out by maintainingthe temperature at 1,300°-1,500° C. for 2-5 hours or by graduallyraising the temperature at a rate of 25° C./hr within the temperaturerange of 1,300°-1,500° C. and then the secondary firing is carried outby maintaining the temperature at 1,650°-1,850° C. for more than onehour. The thus obtained polycrystalline alumina sintered body contains0.001-0.05% of MgO, 0.001-0.05% of La₂ O₃ and 0.001-0.05% of Y₂ O₃ andhas a weight ratio of MgO/La₂ O₃ +Y₂ O₃ of 0.5-2, an in-linetransmission of more than 40% and an excellent mechanical strength. Inparticular, the case when each content of La₂ O₃ and Y₂ O₃ is0.001-0.01%, is preferable. When one of MgO, La₂ O₃ and Y₂ O₃ in thesintered body is less than 0.001%, it is difficult to uniformly dispersethe additive, so that the effect of the additive for uniformly causingthe densification and the grain growth are not developed. Furthermore,when even one of MgO, La₂ O₃ and Y₂ O₃ present in the sintered body ismore than 0.05%, the second phase is formed and the transmission,particularly the in-line transmission, is reduced due to scattering oflight and such a case is not preferable.

When the weight ratio of MgO/La₂ O₃ +Y₂ O₃ is beyond the range of 0.5-2,as mentioned above the additives having different activity affecting thegrain growth of alumina grains are nonuniformly dispersed, so thatalumina grains grow nonuniformly and therefore the grain size in thesintered body is apt to become uneven and the mechanical strength andthe thermal shock resistance are lower. When the oxygen concentration inthe sintering atmosphere is higher than 10⁻¹⁵ atm., oxygen atoms arecaught in pores in the sintered body and their removal is difficult, sothat no dense sintered body is obtained and therefore the transmissionis reduced. When the oxygen concentration is lower than 10⁻²⁵ atm., Al₂O₃ is reduced to form lower oxides, such as AlO, etc., and alumina isreadily volatilized. Accordingly, the oxygen concentration in the firingatmosphere must be within the range of 10⁻¹⁵ -10⁻²⁵ atm. The in-linetransmission in the present invention means a ratio of intensity of thetransmitted light to the incident light when light of a wavelength of0.6 μm is entered by using a double beam-type spectrophotometer withrespect to a sample having a cross-sectional area of 10×10 mm and athickness of 0.5 mm, which has been obtained by polishing the sinteredbody.

The in-line transmission and grain size of a polycrystalline translucentalumina sintered body of the present invention and the lamp efficiencywhen the sintered body is used as an envelope for a 400 W high pressuresodium lamp are shown in the following table. For comparison, theproperties of alumina sintered bodies beyond the composition range ofthe present invention are shown together in the table.

                                      TABLE                                       __________________________________________________________________________                                      Grain size                                         Amount of additives    In-line                                                                           (μm)          Lamp                              in sintered body       trans-                                                                            Average                                                                            Grain size                                                                          Bending                                                                             effi-                             (wt %)     Weight ratio                                                                              mission                                                                           grain                                                                              distribu-                                                                           strength                                                                            ciency                     Sample No.                                                                           MgO                                                                              La.sub.2 O.sub.3                                                                  Y.sub.2 O.sub.3                                                                   MgO/(La.sub.2 O.sub.3 + Y.sub.2 O.sub.3)                                                  (%) size tion  (kg/mm.sup.2)                                                                       (Lm/W)                     __________________________________________________________________________    Present                                                                            1 0.001                                                                            0.001                                                                             0.001                                                                             0.5         63  68   40-90 34    129                        inven-                                                                             2 0.005                                                                            0.003                                                                             0.001                                                                             1.25        54  50   30-80 39    128                        tion 3 0.005                                                                            0.002                                                                             0.005                                                                             0.71        60  53   25-80 38    128                             4 0.005                                                                            0.005                                                                             0.005                                                                             0.5         67  49   25-75 40    130                             5 0.01                                                                             0.0025                                                                            0.0025                                                                            2           44  47   30-70 41    127                             6 0.01                                                                             0.005                                                                             0.005                                                                             1           69  43   25-60 37    130                             7 0.01                                                                             0.01                                                                              0.01                                                                              0.5         72  41   30-55 40    133                        Compar-                                                                            8 0.01                                                                             --  --  --          31  28   15-40 45    120                        ative                                                                         sample                                                                             9 0.05                                                                             0.1 0.1 0.25        65  80    20-130                                                                             21    128                        __________________________________________________________________________

The grain size in the above table was determined by measuring the longaxial diameter through microscopic observation of the surface of thesintered body. When the cross-section of the sintered body was observed,the crystal grain size on the surface of the sintered body wassubstantially the same as the crystal grain size in the inner portion.The bending strength was calculated as an average value of ten samplesof the four point bending strength measured under the condition of anouter span of 30 mm and an inner span of 10 mm with respect to a sampleof 4×3×40 mm.

As seen from the above table, all the samples according to the presentinvention have an in-line transmission of more than 40%, accordingly thelamp efficiency is excellent and the grain size is relatively uniform,so that the mechanical strength is high. In the comparative samplecontaining only MgO, the transmission is not satisfied and when thissample is used as an envelope for a lamp, the lamp efficiency is poor(Sample No. 8). In the sample No. 9 wherein La₂ O₃ and Y₂ O₃ arecontained in an amount of 0.1% respectively and the weight ratio ofMgO/La₂ O₃ +Y₂ O₃ is 0.25, the grain size is very uneven, so that themechanical strength is very poor and when 10 pieces of 400 W highpressure sodium lamps were produced by using this sample, occurrence ofcracks was observed in one envelope upon sealing, while in the sampleNos. 1-7 and comparative sample No. 8, no occurrence of cracks wasobserved.

As seen from the above explanation, the polycrystalline translucentalumina sintered body according to the present invention contains Y₂ O₃and La₂ O₃ as the additive in addition to already used MgO, so that thefiring can be carried out at a lower temperature and the content of MgO,La₂ O₃ and Y₂ O₃ and the weight ratio of MgO/La₂ O₃ +Y₂ O₃ areparticularly defined, so that the second phase is not substantiallyformed in the crystal grain boundary and therefore the in-linetransmission is excellent and alumina crystal grain size is uniform, sothat the mechanical strength is high, so that when a polycrystallinealumina sintered body according to the present invention is used as anenvelope for a high pressure vapor discharge lamp, such as a highpressure sodium lamp and the like, the lamp efficiency is high and saidenvelope can endure large thermal shock when sealing the lamp orlighting on and off. Thus, the present invention has high merit and isvery commercially useful.

What is claimed is:
 1. A polycrystalline translucent alumina sinteredbody in which the content of MgO is 0.001-0.04% by weight, the contentof each of La₂ O₃ and Y₂ O₃ is 0.001-0.01% by weight, a weight ratio ofMgO/La₂ O₃ +Y₂ O₃ is 0.5-2, and an in-line transmission is more than40%.
 2. In a high pressure vapor discharge lamp having an envelope, theimprovement wherein the envelope comprises polycrystalline translucentalumina in which the content of MgO is 0.001-0.04% by weight, thecontent of each La₂ O₃ and Y₂ O₃ is 0.001-O.01% by weight, a weightratio of MgO/La₂ O₃ +Y₂ O₃ is 0.5-2, and an in-line transmission is morethan 40%.